FIG. 1 represents schematically a known manner of measuring the position of the walls of a blood vessel. This figure shows an ultrasonic transducer 1 placed above the skin 2 of a subject, which transducer 1 faces an artery 3 displayed in cross-section. The transducer 1 is controlled by an electronic circuit so as to emit an ultrasonic pulse 4, to receive ultrasonic echos resulting from the reflection of that pulse on the artery-tissue and artery-blood interfaces, and to create an echo signal in response thereto. Depending on the frequency of the ultrasonic transducer, this echo signal can represent four distinct echos 5, 6, 7 and 8, or only two echos corresponding respectively to a combination of the echos 5 and 6, and to a combination of the echos 7 and 8.
The movement of each interface is determined in the following manner. The transducer 1 emits a pulse 4 with a repetition frequency of generally between 10 Hz and 5 kHz. In order to follow the position of the echos, which are detected by the transducer after a delay which depends on the position of each interface, a temporal window of fixed size is used to define a time interval in which the echos are awaited, and which is adjusted, after each cycle, so that the echos would be detected in the centre of that window if the interfaces were immobile.
Knowledge of the temporal position of each interface as well as the propogation speed of the pulse in the blood and the tissue makes it possible, by measuring the interval, to determine the change of the inner diameter and the thickness of the anterior and posterior walls of the blood vessel 3 with time.
FIG. 1 is a schematic diagram only. In practice, the echos resulting from reflection of an impulse on the anterior and posterior walls of the blood vessel 3 are not as pure, but have a much more complex form as is shown by the elemental echos E.sub.ant and E.sub.post in FIG. 2. This deformation results from the fact that the ultrasonic pulse traverses tissues having different characteristics and from the fact that the interface between the wall of a blood vessel and the surrounding tissue is not as clearly defined as, for example, the interface between a metal plate and the surrounding air.
The position of the interfaces causing these echos, notably in the medical domain, thus cannot be directly and automatically determined from the form of the echo signal.
Several techniques may be used for detecting the position of the moving interfaces.
According to a first technique, the position of the interfaces is manually determined. The user displays the echo signal on an oscilloscope or another display means and choses a particular point on the echo signal onto which the echo tracker must lock. A second technique consists of processing the echo signal so as to suppress the noise, only keeping the part of the signal resulting from the reflection of the ultrasonic signal on each interface. In this technique the echo signals to be studied are firstly digitized and stored in real time, and then subsequently treated.
Swiss patent application no. 2871/91 describes a third technique in which an ultrasonic measuring apparatus emits a first ultrasonic pulse towards a blood vessel, converts the echo signal created from the echos detected by the transducer into a series of digital values (created during the opening of the temporal window) which are then stored. In an initialisation stage, these stored digital values are treated so as to select a reference point in each elemental echo of the echo signal, determine for each elemental echo the temporal position of each interface producing that elemental echo, and calculate for each elemental echo the temporal interval between the position of the reference point of that elemental echo and the temporal position of the interface obtained by the processing.
In parallel with this treatment, an assimilation phase occurs in which the digital values resulting from the detection of echo signals from subsequent ultrasonic pulses are treated so as to track the temporal position of the reference points from each echo signal. There follows then an acquisition phase in which the temporal position of each interface corresponding to each elemental echo of echo signals resulting from ultrasonic pulses subsequent to the assimilation phase are followed and memorized. Finally, an exploitation stage occurs during which the data memorized in the acquisition phase is used to provide information to the user, such as displaying the diameter of the blood vessel as a function of time.
It can be seen that the accurate analysis of the form of each echo signal, and in particular the form of its elemental echos, is an important part of each of the above-described techniques.
As shown in FIG. 2, however, the accuracy of this analysis is impeded by the fact that the elemental echo E.sub.post usually has a smaller magnitude than that of the elemental echo E.sub.ant. This results from the greater distance within the subject through which the portion of the ultrasonic pulse reaching the posterior wall of the blood vessel 3, and the echos resulting therefrom, must travel. These greater distances arise both because the posterior wall of the blood vessel 3 is further away from the transducer 1 than the anterior wall, and also because of errors in the alignment of the transducer 1 and the reflective surfaces of the blood vessel 3 which result in the echos therefrom not being reflected directly back to the transducer 1.
Accordingly, it is desirable to have a ultrasonic measuring system providing characteristics which may be set so as to optimize the amplitude of a first elemental echo, and that may be altered to an optimum condition for a second elemental echo.
U.S. Pat. No. 4,451,797 discloses an automatic gain controller for a pulsed system used in the non-destructive testing of pipe walls. In the ultrasonic inspection system described therein, a probe emits an ultrasonic pulse, and receives the resulting echos from the near and far surfaces of the pipe. An agc amplifier is connected to the probe and produces an echo signal representative of the echos received by the probe. Following the emission of the pulse, a first circuit is connected to the output of the agc amplifier. When the echo from the near wall of the pipe is received, the magnitude of the echo signal is compared to a reference voltage and the difference used to set the amplification of the agc amplifier. The amplitude of the part of the echo signal corresponding to the echo from the near wall is thus optimized.
After the receipt of the echo from the near wall but before the receipt of the echo from the far wall, a second circuit is connected to the output of the agc amplifier. When this latter echo is received by the probe, the magnitude of the echo signal is compared to a different reference voltage, the difference being used to set the amplification of the agc amplifier to a new level. The amplitude of the part of the echo signal corresponding to the echo from the far wall is thus also optimized.
In this automatic gain controller, the amplification of each echo signal (resulting from a single ultrasonic pulse) is adjusted between the receipt of the near wall echo and the far wall echo. In many applications, however, this type of gain adjustment is not possible, as is the case when the temporal movement of the position of the walls of a blood vessel is followed.
Due to the capacitive and inductive properties inherent in any gain control circuit, a certain settling time is required if the amplification of a signal is to be changed. It has been found experimentally that for many such amplifier circuits, a settling time of several .mu.sec is required. Ultrasonic waves travel within a subject at a speed of approximately 1540 m/sec. Whilst a femoral or carotid artery can have a diameter of as much as 1 cm, other human arteries may be as small as 4 mm, so that in the latter case the elemental echos E.sub.ant and E.sub.post may only be separated by 5.2 .mu.sec for a stationary artery. As a subject's arteries are not stationary, but in fact move between pulses from the transducer 1, the time between the calculated moment at which the transducer gain can be adjusted and the actual moment at which the echo from the posterior wall of the blood vessel 3 is received is actually less than this. It can thus be seen that using the above described gain control system, insufficient time is available between elemental echos within which to change the amplitude of the echo signal.